1. Field of the Invention
The present invention relates to a method of manufacturing an optical waveguide for use as an optical line, and a method of manufacturing an opto-electric wiring board which comprises optical waveguides and electric wires mixedly formed thereon.
2. Description of the Related Art
Generally, electronic parts such as LSI (Large Scale Integrated Circuit) are mounted on electric wiring boards formed with electric wires for assembly into a variety of electronic devices. An electronic device implements a predetermined function by transmitting electric signals through electric wires which interconnect respective electronic parts. Due to unwanted noise possibly generated during its operation, an electronic device adversely affects other electronic devices located nearby which would suffer from malfunctions, degraded performance, and the like. Also, in the electronic device, electric signals may experience a delay associated with a time constant determined by capacitive and resistive components contained in electric wires.
In recent years, on the other hand, an increasing prevalence has been achieved by optical communication technologies which employ optical signals as information transmission media for realizing a larger capacity and a higher speed of communications. To respond to the trend, optical communication devices have been aggressively under development for building optical communication networks, such as an optical exchanger, an optical interconnector, and the like.
In optical communication devices, optical waveguides formed on insulating substrates are used as optical lines for transmitting optical signals. An optical communication device does not adversely affect other optical communication devices or electronic devices located nearby because it does not generate noise in operation as is the case with an electronic device. In addition, optical lines are free from a loss of high frequency signals due to the skin effect of conductors. To take advantages of these features, a proposed device comprises optical lines which are partially substituted for electric wires to use optical signals instead of electric signals, thereby reducing adverse effects otherwise exerted on other surrounding electronic devices and realizing a higher speed of signal transmission.
The above-mentioned device employs an opto-electric wiring board which has electric wires and optical lines mixedly laminated thereon. However, an electric wiring board generally has a multiplicity of electric wiring layers for purposes of mounting electronic parts at a high density, so that the electric wiring board is formed with ruggedness on the surface due to the multiple wiring layers. If an optical waveguide is directly formed on the rugged surface for routing an optical line, the resulting optical waveguide will be distorted to cause a large transmission loss of an optical signal. For this reason, a need exists for techniques for laminating electric wires and optical lines on the same board while reducing a transmission loss of optical signals.
An example of such technique is disclosed, for example, in Japanese Patent Application Laid-open No. 2001-15889. In the following, a method of manufacturing an opto-electric wiring board according to a first prior art will be described with reference to FIGS. 1A to 1E.
In the manufacturing method according to the first prior art, optical waveguide resin layer (optical line layer film) 103 is first formed on first supporting substrate 101 made of silicon through strip layer 102 made of Cr, Cu or the like for routing an optical line, as illustrated in FIG. 1A. Optical waveguide resin layer 103 is comprised of a lower cladding layer, an upper cladding layer, and a core layer sandwiched between the lower and upper cladding layers for transmitting an optical signal. First supporting substrate 101 is used to prevent thin optical waveguide resin layer 103 from deforming into a curled layer.
Next, as illustrated in FIG. 1B, strip layer 102 is dissolved to remove optical waveguide resin layer 103 from first supporting substrate 101.
Next, as illustrated in FIG. 1C, optical waveform resin layer 103 is adhered to second supporting substrate 105 made of glass using first adhesive 104. In this event, a known laminator, for example, is used to adhere optical waveguide resin layer 103 to second supporting substrate 105. Second supporting substrate 105 plays the same role as first supporting substrate 101.
Next, as illustrated in FIG. 1D, optical waveguide resin layer 103 adhered on second supporting substrate 105 is adhered to electric wiring board 107 which is a polyimide multi-layered wiring board formed with electric wires (not shown) on the surfaces, using second adhesive 106 made of a denaturation polyimide resin. Second adhesive 106 has a larger adhesion strength than first adhesive 104.
Finally, as illustrated in FIG. 1E, second supporting substrate 105 is stripped from optical waveguide resin layer 103 together with first adhesive 104, thereby completing opto-electric wiring board 108 which has optical waveguide resin layer 103 formed on electric wiring board 107.
Manufactured through the foregoing steps is opto-electric wiring board 108 which has optical waveguide resin layer 103 for providing optical lines and electric wires (not shown) formed on the same electric wiring board 107. In the structure as described above, since optical waveguide resin layer 103 is mounted on electric wiring board 107 through second adhesive 106, the optical lines will not be adversely affected by ruggedness on the surface of electric wiring board 107. This results in a reduction in transmission loss of optical signals.
A method of manufacturing an optical waveguide which can be mounted on an opto-electric wiring board is disclosed, for example, in Japanese Patent Application Laid-open No. 2001-154051 as a method of manufacturing an opto-electric wiring board according to a second prior art. In the following, the manufacturing method according to the second prior art will be described with reference to FIGS. 2A to 2D.
In the second prior art, first, intermediate cladding layer 113 and upper cladding layer 112 each made of a polyimide fluoride film are previously laminated sequentially on copper substrate 114, as illustrated in FIG. 2A. Intermediate cladding layer 113 is patterned into a predetermined shape using a known photolithography technique. A copper thick film is filled in remaining intermediate cladding layer 113 on upper cladding layer 112. Supporting substrate 111 made of aluminum is adhered on upper cladding layer 112.
Next, after copper substrate 114 and copper thick film 115 are removed, respectively, as illustrated in FIG. 2B, core layer 116 made of a polyimide fluoride film is formed in regions from which copper thick film 115 has been removed, as illustrated in FIG. 2C.
Next, as illustrated in FIG. 2D, lower cladding layer 117 made of a polyimide fluoride film is formed to cover intermediate cladding layer 113 and core layer 116. Finally, supporting substrate 111 is removed to complete optical waveguide 118. This optical waveguide 118 is adhered to an electric wiring board, separately fabricated, to provide an opto-electric wiring board.
The first mentioned method of manufacturing an opto-electric wiring board according to the first prior art has a problem of a long manufacturing time and a resulting increase in cost due to the requirement of a plurality of types of supporting substrates which include first supporting substrate 101 for forming optical waveguide resin layer 103 that provides optical lines (optical waveguide), and second supporting substrate 105 for holding optical waveguide resin layer 103 stripped from first supporting substrate 101.
Specifically, in the method of manufacturing an opto-electric wiring board according to the first prior art, first supporting substrate 101 made of silicon is used for forming optical waveguide resin layer 103, as illustrated in FIG. 1A, and second supporting substrate 105 made of glass is used for again supporting optical waveguide resin layer 103, as illustrated in FIG. 1C.
The use of two supporting substrates requires a working step for stripping optical waveguide resin layer 103 from first supporting substrate 101 as well as a working step for again adhering stripped optical waveguide resin layer 103 to second supporting substrate 105, and a working step for stripping optical waveguide resin layer 103 from second supporting substrate 105. Thus, an additional time required for manufacturing the opto-electric wiring board causes an increase in the manufacturing cost.
Also, in the method of manufacturing an opto-electric wiring board according to the first prior art, thin optical waveguide resin layer 103 is susceptible to deformation into a curled layer when it is stripped from first supporting substrate 101, giving rise to a problem that optical waveguide resin layer 103 is hard to handle during the manufacturing process.
As illustrated in FIG. 1B, thin optical waveguide resin layer 103 is curled when it is stripped from first supporting substrate 101, so that curled optical waveguide resin layer 103 must be reshaped into a flat layer before the step of adhering optical waveguide resin layer 103 to second supporting substrate 105, as illustrated in FIG. 1C. However, a long time is consumed for the adhesion particularly when optical waveguide resin layer 103 is reshaped into a flat layer with difficulties.
The method of manufacturing an optical waveguide according to the second prior art, on the other hand, has a problem of a residual stress produced in the optical waveguide because of the difference in thermal expansion coefficient between materials of the supporting substrate and optical waveguide.
In the method of manufacturing an optical waveguide according to the second prior art, after upper cladding layer 112 is adhered to supporting substrate 111 made of aluminum as illustrated in FIG. 2B, core layer 116 and lower cladding layer 117 each made of a polyimide fluoride film are formed respectively, as illustrated in FIGS. 2C and 2D. The polyimide fluoride film is coated, for example, with a polyamide acid solution, and then thermally treated for imidization. Subsequently, the resulting film is cooled down to a room temperature for curing. As a result, a stress remains in optical waveguide 118 due to a difference in thermal expansion coefficient between aluminum and polyimide fluoride film. The stress makes optical waveguide 118 more susceptible to cracking and the like, resulting in a lower long-term reliability of optical waveguide 118.